Draft: Boehm/precomputation

Closes #51.

Adds the option to assemble local matrices via computeAndStoreLocalMatrices(). Adds the option to load a local matrix instead of assembling it on-the-fly during Apply(). With adaptive Galerkin coarsening in mind, this is done in a selective manner: a map of PrimitiveIDs can be passed to the operator; only for the specified primitives the local matrices will be assembled and applied from memory.

added 2 commits

• 8ce0c2e9 - revert experimental change to indexing
• c4e49972 - remove pspg generation

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mentioned in issue #51

changed the description